1. Field of the Invention
This invention pertains to electromagnetic surveying and to electrodes useful in magnetotelluric applications, especially in the environment of low frequency reception.
2. Description of the Prior Art
It is well-known that electromagnetic fields impinge on the earth's surface at all times and create both electrical fields and magnetic fields that are detectable utilizing proper techniques and apparatus.
Electric field and magnetic field detection at spaced apart locations can be important in electrical conductivity measurements for many applications. One important application is for making electromagnetic measurements useful in surveying for presence of hydrocarbon deposits located beneath very dense and rigid lithographic structures. For example, an oil or gas deposit located beneath a granite structure is virtually impossible to detect using conventional seismic techniques, but is detectable by measuring the voltage across two electrodes or "antennas" distantly placed. A typical electric dipole antenna electrode structure used in electromagnetic surveying comprises two electrodes buried in the ground and connected to wires that are fed to an electrometer amplifier for measuring voltage. In such a system there is an electronic-to-ionic contact between the electrodes of the antenna pair and the brines in the ground. Noise studies have been made on these antennas by G. Petian and A. Dupis, as reported in 28 Geophysical Prospecting at page 792 (1980) and indicate that the antenna noise is dominated by the electrode noise at frequencies below 1 Hz. This frequency limitation is important since high penetration or large depth studies can only be conducted at low frequencies. At higher frequencies, the electrical/ionic conducting barrier at the electrodes is small. However, at low frequency, the ions accumulate at a metallic electrode and create a very noisy electrode.
One approach to reducing noise is to use a solid metal that has a large surface area for slow ionic build up. A lead sheet of approximately one square foot area has been employed by being buried in the ground, connected to the antenna wire by an alligator clip, then saturated with a saline solution and stabilized for a day before measurements are made. This arrangement, although cumbersome, has allowed magnetotelluric measurements down to frequencies as low as 10.sup.-3 Hz. However, the area cannot be increased without limit because there are irreversible hydration or oxidation reactions that will eventually fully oxidize such a high-surface-area metal electrode. This oxidation will itself induce electrical noise.
A second type of electrode that has been used is the Cu-CuSO.sub.4 "pot", comprising a copper wire in an electrolyte of CuSO.sub.4 contained in a porous "pot" of inert material through which the ground water seeps. The observed noise of this type electrode is associated with current flow and appears to be in the charge flow across the metal-to-electrolyte interface and between the electrolyte and the ground water located in the wall pores of the porous pot.
None of the prior art structures that are known have low noise operating characteristics satisfactory for very low frequency application as hereinafter described.
Therefore, it is a feature of the present invention to provide an improved electrode for low frequency magnetotelluric use that is large in area and made of materials that are both electronically and ionically conducting and that are chemically compatible to the high salinity compounds found in solution in soils and stable therein.
It is another feature of the present invention to provide an improved method of making electrical conductivity measurements using the type of individual electrode described below.
It is still another feature of the present invention to provide an improved electrode of the type described below wherein the electronically and ionically conducting materials includes an intercalation or insertion compound.